GMR nanowire sensors

ABSTRACT

A magnetic position sensor having at least one magnetic field sensor including a solidified layer of GMR nanowire carrier fluid formed on a substrate material. The solidified layer of carrier fluid has (i) discrete GMR nanowires each having a diameter of less than about 0.5 um and a length less than about 250 um; and (ii) a concentration of GMR nanowires in the dried layer between about 0.001 and about 10 percent by weight of the solution. The position sensor further includes a detection circuit capable of detecting a change in resistance of the magnetic field sensor.

This application claims the benefit under 35 USC 119(e) of U.S.Provisional Application Ser. No. 61/622,605 filed Apr. 11, 2012, whichis incorporated by reference herein in its entirety.

This invention was made with government support under grant numberNNX10AI40H awarded by the National Aeronautics & Space Administration.The government has certain rights in the invention.

BACKGROUND OF INVENTION

Magnetoresistance (MR) is the property of a material to change itselectrical resistance in the presence of a magnetic field. Magneticsensors based on the MR effect can measure the strengths of magneticfields or the relative direction of the fields. One of the mostsignificant applications of MR sensors is their incorporation into readheads for magnetic recording devices. Five distinct types of MR areordinary magnetoresistance (OMR), anisotropic magnetoresistance (AMR),giant magnetoresistance (GMR), tunneling magnetoresistance (TMR) andcolossal magnetoresistance (CMR).

There are two types of GMR geometries, which are named current in plane(CIP) and current perpendicular to the plane (CPP). Each geometry has adifferent quantum mechanical effect that changes the probability ofconduction electrons scattering throughout their layers and consequentlychanging the electrical resistance of the GMR material. CIP GMRdescribes a geometry whereby the magnetic field must be applied in thesame direction as the current flow for a change in resistance to beobserved in the material and the change in the mean free path ofconduction electrons determines the amount of resistance change.Alternatively, CPP GMR refers to a geometry typically having multilayersin high aspect ratio configurations. With CPP GMR, a magnetic field mustbe applied perpendicular to the current flow for resistance changes tobe observed. The magnetic field alters the magnitude of the spin-flipdiffusion length of conduction electrons, influencing the resistancechange of the GMR material. CPP GMR performance is partially a functionof the number of alternating material layers with a higher number oflayers giving higher changes in resistances in magnetic fields.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates one embodiment of a one dimensional position sensor.

FIG. 2 illustrates one method of forming nanowires for use in theposition sensors.

FIGS. 3A and 3B further illustrate methods of forming nanowires.

FIG. 4 illustrates a second embodiment of a one dimensional positionsensor.

FIG. 5 illustrates a third embodiment of a one dimensional positionsensor.

FIGS. 6A and 6B illustrate one embodiment of a push-button sensor.

FIGS. 7A and 7B illustrate a second embodiment of a push-button sensor.

FIGS. 8A and 8B illustrate a third embodiment of a push-button sensor.

FIGS. 9A to 9C illustrate a potentiometer type device.

FIG. 10 illustrates a two dimensional position sensor.

FIG. 11 illustrates sensor elements formed on a removable adhesivesubstrate.

FIG. 12 illustrates one embodiment of a sensor detection circuit.

FIG. 13 illustrates one embodiment of a ferromagnetic object sensor.

FIG. 14 illustrates one embodiment of a flux concentrator structure.

FIG. 15 is an SEM image of a solidified layer of GMR nanowires.

DETAILED DESCRIPTION OF SELECTED EMBODIMENTS

One embodiment of the present invention is a magnetic position sensor 1such as illustrated in FIG. 1. The position sensor 1 includes at leastone, and more typically a series of, magnetic field sensors 2 formed ona substrate 10. In this embodiment, the magnetic field sensors 2 areshaped as a serpentine line 3 of GMR nanowires formed upon substrate 10.The ends of serpentine line 3 are connected to conductors 7 and 8 whichultimately will connect the magnetic field sensor 2 to a detectioncircuit. Although more detailed examples of detection circuits areprovided below, the detection circuit will generally measure theresistance of the sensors. Two common resistance measurements include:first, by changing a resistance into a corresponding voltage formeasurement purposes; and secondly, by placing the sensor in a RCcircuit and measuring the charging and discharge rate of the capacitor.

In this embodiment, the serpentine lines 3 are approximately 0.5 mm to 1mm wide, but may be narrower or wider, e.g., about 0.01 to 10 mm. Thelines 3 are formed by adhering (e.g., drying or curing) a GMR nanowirecarrier solution onto the substrate 10 in the pattern of the serpentinelines. The GMR nanowire carrier liquid adhered to or dried upon asubstrate may be referred to as a solidified layer of GMR nanowires. Asused herein, “nanowire” or “nanostructure” generally refers to smallstructures, at least one dimension of which (i.e., width or diameter) isless than 1000 nm, more typically, less than 500 nm or 100 nm or 50 nm.In various embodiments, the width or diameter of the nano structures arein the range of 10 to 40 nm, 20 to 40 nm, 5 to 20 nm, 10 to 30 nm, 40 to60 nm, or 50 to 70 nm. The length of a nanowire may vary in differentembodiments from less than 1 micron to several hundred microns. In manyembodiments, the GMR nanowires in the carrier solution may have adiameter of less than about 0.5 um and a length less than about 200 um.More preferably, the GMR nanowires will have a diameter between about 10nm and about 200 nm and a length between about 1 um and about 100 um. Ofcourse, other specialized embodiments may have nanowire diametersgreater than 0.5 um and lengths greater than 200 um. In manyembodiments, the GMR nanowires will have an aspect ratio (i.e., lengthover diameter) of at least three and more preferably at least about 10,and still more preferably at least about 50. The term “nanowire”includes any elongated structure such as nanorods (solid nanostructures) and nanotubes (hollow nanostructures), whether thecross-section is generally round or some other shape.

In many embodiments, the concentration of GMR nanowires within thecarrier solution may be between about 0.005 and about 10 percent byweight of the solution, or any subrange there between. In certainembodiments, the concentration of GMR nanowires within the carriersolution is between about 0.01 and about 1.5 percent. Naturally, otherembodiments could be less than 0.01 or greater than 10 percent. When thedried GMR nanowire layer is formed on the substrate, the nanowires willgenerally be present in a quantity ranging from about 1,000 nanowiresper mm² to about 50,000 nanowires per mm² or any subrange there between.As a general principle, the longer the nanowires, the fewer needed toform a connective nanowire network for magnetoresistance measurement.Again, particular embodiments could employ fewer than 100 nanowires permm² or more than 5,000,000 mm² (or any subrange between these values).

The GMR nanowires are often suspended or dispersed in a carrier liquidto facilitate their deposition on the substrate. Typically, the GMRnanowires are at least temporarily suspended in the carrier fluid.However, the nanowires only need remain suspended long enough to beapplied to the substrate material. Preferably, the carrier fluid shouldbe homogeneous, i.e., the GMR nanowires should be suspended uniformly inthe carrier solution. It is understood that, as used herein,“deposition” and “coating” are considered interchangeable. Any number offluids may be used as the nanowire carrier fluid. For example, anynon-corrosive liquid in which the GMR nanowires can form a stabledispersion may be used. Preferably, the GMR nanowires are dispersed inwater, an alcohol, a ketone, ethers, hydrocarbons or an aromatic solvent(benzene, toluene, xylene, etc.). More preferably, the liquid isvolatile, having a boiling point of no more than 200° C., no more than150° C., or no more than 100° C.

In addition, the GMR nanowire carrier fluid (or “dispersion”) maycontain binders and other additives to control viscosity, corrosion,adhesion, and nanowire dispersion.

A binder may be any material or substance that holds or draws othermaterials together to form a cohesive whole. Binders may include gellingor thickening agents along with viscosity modifiers. Examples ofsuitable binders include, but are not limited to, carboxy methylcellulose (CMC), 2-hydroxy ethyl cellulose (HEC), hydroxy propyl methylcellulose (HPMC), methyl cellulose (MC), poly vinyl alcohol (PVA),tripropylene gylcol (TPG), and xanthan gum (XG).

Examples of suitable viscosity modifiers include biopolymers, such ashydroxypropyl methyl cellulose (HPMC), methyl cellulose, xanthan gum,tragacanth gum, carboxy methyl cellulose, and hydroxy ethyl cellulose;acrylic polymers, such as, sodium polyacrylate; and water-solublesynthetic polymers, such as, polyvinyl alcohol.

Surfactants are compounds that lower the surface tension of a liquid,the interfacial tension between two liquids, or that between a liquidand a solid. Surfactants may act as detergents, wetting agents,emulsifiers, foaming agents, and dispersants. Examples of surfactantsmay include ethoxylates, alkoxylates, ethylene oxide and propylene oxideand their copolymers, sulfonates, sulfates, disulfonate salts,sulfosuccinates, phosphate esters, and fluorosurfactants (e.g., Zonyl®by DuPont). More particular examples of suitable surfactants includeZonyl® FSN, Zonyl® FSO, Zonyl® FSH, Triton (x100, x114, x45), Dynol(604, 607), n-Dodecyl b-D-maltoside and Novek.

In one example, the nanowire dispersion may include, by weight, from0.0025% to 0.1% surfactant (e.g., a preferred range is from 0.0025% to0.05% for Zonyl® FSO-100), from 0.02% to 4% viscosity modifier (e.g., apreferred range is 0.02% to 0.5% for HPMC), from 94.5% to 99.0% solventand from 0.01% to 1.4% GMR nanowires.

Solvents help adjust the curing rate and viscosity of the solution inits liquid state. The solvents evaporate off as the solution dries.There are two types of carrier solutions: solvent-based and water-based.Examples of suitable solvents include alcohols, ketones, ethers,hydrocarbons or aromatic solvents (benzene, toluene, xylene, etc.) asmentioned above. The ratios of components of the dispersion may bemodified depending on the substrate and the method of application used.The viscosity range of the GMR nanowire carrier liquid may vary greatlydepending on the method of application, For inkjet printingapplications, the viscosity can range from about 1 to about 40 cP; forgravure printing about 40 to about 200 cP; and for screen printing, over1000 cP. Thus the viscosity may generally range from about 0.3 to about2000 cP (or any subrange in between). One preferred viscosity range forthe nanowire dispersion is between about 1 and 40 cP (or any subrangethere between). Certain embodiments of the carrier fluid may includeflux concentrator agents such as nanoparticles from an alloy consistingof cobalt, magnetite, or iron. Additional information on carrier fluidsmay be found in U.S. Pat. No. 8,174,667 which is incorporated byreference herein in its entirety

The GMR nanowires themselves are typically constructed by alternatingferromagnetic and non-magnetic conductive layers. Typically, these willbe CPP GMR nanostructures In certain embodiments, the ferromagneticconductive layers are less than 100 nm (in one example 15 nm) inthickness and non-magnetic conductive layers are less than 50 nm (in oneexample 5 nm) in thickness. The GMR nanowires comprise at least 2alternating ferromagnetic and non-magnetic conductive layers, and moretypically, will comprise tens, hundreds, or even thousands ofalternating layers. While many embodiments of the GMR nanowires willhave at least 5, 10, 25, or 50 alternating layers, certain embodimentscould have fewer alternating layers. Example compounds forming theferromagnetic layer may include Co, CoFe, CoNiFe, CoNi, CoNiFeCr, CoCr,CoNiCr, NiFe, NiCo, or NiCoCr. Example materials forming thenon-magnetic conductive layer include Cu, Ag, Au or alloys of thesemetals. One particular example of a GMR structure is disclosed in U.S.Pat. No. 7,016,168 which is incorporated by reference herein in itsentirety. In this example, the GMR structure includes a seed layer; apinning layer formed of an antiferromagnetic material formed on the seedlayer; and a synthetic antiferromagnetic pinned layer formed on thepinning layer. The pinned layer further comprises: a first ferromagneticlayer; an antiferromagnetically coupling layer formed on the firstferromagnetic layer; a second ferromagnetic layer formed on the couplinglayer; and wherein the magnetizations of the first and secondferromagnetic layers are antiparallel. In this embodiment, the firstferromagnetic layer is a layer of CoFe formed to a thickness betweenapproximately 20 and 60 angstroms; the second ferromagnetic layer is alayer of CoFe formed to a thickness between approximately 20 and 60angstroms; and the coupling layer is a layer of Ru formed to a thicknessbetween approximately 6 and 9 angstroms. Naturally, the above is merelyone example of an acceptable GMR structure and many conventional orfuture developed GMR structures may be employed in the presentinvention. Likewise, the GMR nanowires could be formed by any number ofconventional or future developed fabrication techniques. Oneillustrative example suggested in FIG. 3A is a method including thesteps of (a) providing a substrate such as a anodized aluminum oxide(AAO) membrane 42 with nano size pores, i.e., pores less than about 0.5um in diameter and at least 1 μm in depth; (b) making one side of theAAO membrane conductive by applying a conductive material 43, (c)forming alternating ferromagnetic and non-magnetic conductive layerswithin the pores; and (d) dissolving the AAO membrane in order torelease the GMR nanowires 45.

This method is a template assisted nanowire fabrication technique. Thetemplate (i.e., the AAO membrane) has many nano size pores that serve asa mold for the formation of a single nanowire. AAO membranes are used astemplates in many nanotechnology applications to eliminate the need forexpensive lithographical techniques. The formation of the nanoporous AAOmembranes is done by an electro-chemical process that changes thesurface chemistry of the metal, by oxidation, resulting in an anodicoxide layer. By tightly controlling the pH of the electrolyte andvoltage used during the AAO fabrication, highly ordered arrays ofcylindrical shaped nanopores can be formed with well-regulated porediameters, periodicities and density distributions.

Usually the bottom of the template is made conductive by sputtering on ametal, such as gold, in order to form a base substrate for the formationof nanowires. Although many different materials may be used for thetemplate, e.g. polycarbonate membranes, preferred embodiments includeanodized aluminum oxide membrane templates because they possessed ahigher uniformity of pores at a higher density. For example, thealuminum oxide pore sizes may range from 10 nm-200 nm, with a poredensity of 9×10⁸ to 2×10¹¹ pores/cm², and an aluminum oxide filmthickness of about 15 μm to 150 μm.

As referenced above, the substrate was made conductive by sputtering agold layer and this allows the deposition of nanowires into the templatethrough an electrodeposition or electroplating process. FIG. 2 suggeststhe pores of a membrane template with the conductive substrate servingas a cathode. The membrane is connected as a cathode in a standardelectrochemical setup with the template, anode, cathode, and referenceelectrode positioned in an electrolyte solution containing ions of themetals to be deposited. When a constant DC voltage is applied in theelectroplating process, metal ions from the electrolyte solution willtravel inside the nanopores and bond with the substrate and as thedeposition continues to take place, metal nanowires are formed insidethe pores.

Formation of GMR nanowires typically requires the electroplatingmultilayers of two or more separate metals or alloys within thetemplate. Deposition of multilayers from one electrolyte solution may beaccomplished by choosing metals with high differences in standardelectrode potentials. To overcome the problem of depositing alloysinstead of pure metals in electrodeposited multilayer materials, theelectrolyte solution should contain only traces of metal A ions with ahigh concentration of metal B ions (assuming that metal A has a higherstandard electrode potential than metal B). At an adequately lowpolarization potential the rate of deposition of metal B is high whilethe rate of deposition of metal A is slow because it is controlled bydiffusion. When the potential is at a considerably less negativepolarization potential, only metal A is deposited. As the potential iscycled between the two potentials, multilayers of substantially puremetal A and metal B with traces of metal A are formed, as suggested inFIG. 2.

In this embodiment, a pulsing potential electrodeposition techniquescheme was used to electrodeposit the multilayer GMR nanowires. In oneexample, in order to ensure that the layers that are deposited are flat,a copper base is formed at the bottom of the membrane by keeping thepotential at −0.25 V for 2 hrs. However, in other examples, nanowireshave been produced by keeping the potential at −0.25 V for a muchshorter time, e.g., 15 minutes. After the Cu base is formed, themultilayers are deposited by first keeping the potential at −0.35 Vuntil a total amount of 0.005 C of charge passed through the cathode forcopper layers and then changing the potential to −2.2 V for 0.01 C ofcharge for the alloy (e.g., CoNiFeCu) layers. After deposition of thealloy layer, the potentiostat is transitioned into open circuit toprevent severe alloy dissolution during the copper deposition cycle thatcould damage the interfaces and leave the alloy layer thickness inexact.Following the open circuit interruption, the entire cycle is repeatedover again starting with the copper layer. The length of the nanowiresdepended on the number of cycles performed during electrodeposition. Onaverage, it took 30 s for a copper layer, less than is for an alloylayer, and 5 s for the open circuit stage for a total of 36 s for acomplete cycle. Combing the 2 hrs for the copper base at the beginningof the fabrication process with the deposition of the multilayers, ittook 17 hrs to form GMR nanowires having 1,500 layers.

After GMR nanowires of the selected metals are electrodeposited into thepores of the anodized aluminum oxide (AAO) membrane templates, theconductive bottom layer can be removed and the template can be dissolvedwith strong base freeing the nanowires. After the template is completelydissolved, the nanowires can then be rinsed and suspended in a carriersolution, such as DI water or isopropyl alcohol. When the nanowires areneeded for further fabrication steps, ultrasonication may be used tobetter disperse the nanowires within the carrier solution.

Although electrodeposition within a template is described above, manydifferent techniques could be used to create the nanowires. For example,chemical processes such as chemical vapor deposition (CVD) techniques.Alternatively, physical vapor deposition techniques could be employed,including evaporation (deposition) procedures such as (i) molecular beamepitaxy (MBE), (ii) electron beam epitaxy, or (iii) ion plating.Likewise, sputtering procedures could be used, such as (i) ion-beamsputtering, (ii) RF magnetron sputtering, or (iii) DC magnetronsputtering.

Using a template assisted fabrication method typically produces a singlenanowire per pore of the template. The length of the formed nanowire iseither the same as the thickness of the template or shorter. However, incertain embodiments, it may be disadvantageous to use long nanowires formagnetic sensors due to the method of applying the GMR nanowire carrierfluid. For instance, particle sizes over 2 μm may tend to clog an inkjetprinter. If the length of the nanowires must be shorter, the density ofnanowires must increase for conductivity and, the densities shouldincrease exponentially when the length of the nanowires decreaseslinearly in order to achieve the same degree of connectivity of thenanowires.

As one solution to this difficulty, multiple smaller multilayer GMRnanowires may be created by stacking them inside a single pore assuggested in FIG. 3B. Stacking of the multilayer GMR nanowires can beaccomplished by electrodepositing a sacrificial etch layer 46 (e.g.,gold) between multilayer GMR nanowires. Once the template 47 is removed,the etch layer 46 can be dissolved away leaving the multilayer GMRnanowires 45. FIG. 3B shows (a) a pore with multiple multilayer GMRnanowires separated by the sacrificial etch layer, (b) the templatedissolved, and (c) the sacrificial etch layers removed, creating aseries of smaller nanowires.

In other embodiments, the multilayer GMR nanowires can be fabricatedfrom multiple electrolyte baths, including one electrolyte for theferromagnetic layer and a separate electrolyte bath for the nonmagneticconductive layer. The GMR template can be transferred between bathseither manually or through a robotic process. Likewise, the GMRnanowires may be produced by other techniques which do not necessarilyemploy a template.

As used herein, “substrate” refers to a material onto which the GMRnanowire layer is positioned or formed onto, whether directly orindirectly. The substrate can be rigid or flexible, clear or opaque.Suitable rigid substrates include, for example, silicon, glass,polycarbonates, acrylics, and the like. Suitable flexible substratesinclude, but are not limited to: polyesters (e.g., polyethyleneterephthalate (PET), polyester naphthalate, and polycarbonate),polyolefins (e.g., linear, branched, and cyclic polyolefins), polyvinyls(e.g., polyvinyl chloride, polyvinylidene chloride, polyvinyl acetals,polystyrene, polyacrylates, and the like), cellulose ester bases (e.g.,cellulose triacetate, cellulose acetate), polysulphones such aspolyethersulphone, polyimides, silicones and other conventionalpolymeric films. Additional examples of suitable substrates can be foundin U.S. Pat. No. 6,975,067 which is incorporated by reference herein.

In certain embodiments, the substrate may be functionalized to betterreceive GMR nanowires. Examples of functionalizing compounds may include3-aminopropyltriethoxysilane (APTES), carboxylic acids, orself-assembled monolayers (SAMs). Example SAM materials include oleicacid, tetramethylammonium hydroxide, citric acid, soy lecithin, sodiumdodecyl sulfate, perfluorodecyltrichlorosilane (FDTS),octadecyltrichlorosilane (OTS), polyethylene oxide (PEO), and TritonX-100 (TX-100), and polyvinyl alcohol (PVA). Various conventional orfuture developed techniques may be used to prevent or reduce corrosionof the GMR nanowires which would otherwise reduce magneto-resistanceproperties. For example, a “barrier layer” could be formed on the layerof GMR nanowires adhered to the substrate. “Barrier layer” refers to alayer that reduces or prevents gas or fluid permeation into the GMRnanowire layer. Barrier layers are well known in the art, includingwithout limitation: see, e.g. U.S. Patent Application No. 2004/0253463,U.S. Pat. Nos. 5,560,998 and 4,927,689, EP Patent No. 132,565, and JPPatent No. 57,061,025. Furthermore, a protective film above the GMRnanowires may act as a barrier layer. For example, the protective filmcould be flexible and made of the same material as a flexible substrateon which the GMR nanowires positioned. Examples of protective filmsinclude, but are not limited to: polyester, polyethylene terephthalate(PET), polybutylene terephthalate, polymethyl methacrylate (PMMA),acrylic resin, polycarbonate (PC), polystyrene, triacetate (TAO),polyvinyl alcohol, polyvinyl chloride, polyvinylidene chloride,polyethylene, ethylene-vinyl acetate copolymers, polyvinyl butyral,metal ion-crosslinked ethylene-methacrylic acid copolymers,polyurethane, cellophane, polyolefins or the like; particularlypreferable are PET, PC, PMMA, or TAO because of their high strength.

In other embodiments, GMR sensor structure may include other types ofcorrosion inhibitors, in addition to, or in lieu of the barrier layer asdescribed above. Different corrosion inhibitors may provide protectionto the metal nanowires based on different mechanisms. According to onemechanism, the corrosion inhibitor binds readily to the GMR nanowires,forming a protective film on a metal surface. These are also referred toas barrier-forming corrosion inhibitors. In one embodiment, thebarrier-forming corrosion inhibitor includes certain nitrogen-containingand sulfur-containing organic compounds, such as aromatic triazoles,imidazoles and thiazoles. These compounds have been demonstrated to formstable complexes on a metal surface to provide a barrier between themetal and its environment. For example, benzotriazole (BTA) is a commonorganic corrosion inhibitor for copper or copper alloys. Alkylsubstituted benzotriazoles, such as tolytriazole and butyl benzyltriazole, can also be used. (See, e.g., U.S. Pat. No. 5,270,364.)Additional suitable examples of corrosion inhibitors include, but arenot limited to: 2-aminopyrimidine, 5,6-dimethylbenzimidazole,2-amino-5-mercapto-1,3,4-thiadiazole, 2-mercaptopyrimidine,2-mercaptobenzoxazole, 2-mercaptobenzothiazole, and2-mercaptobenzimidazole.

Another class of barrier-forming corrosion inhibitors includesbiomolecules that show a particular affinity to the metal surface. Theseinclude small biomolecules, e.g. cysteine, and synthetic peptides andprotein scaffolds with fused peptide sequences with affinity for metals;see, e.g. U.S. application Ser. Nos. 10/654,623, 10/665,721, 10/965,227,10/976,179, and 11/280,986, U.S. Provisional Application Ser. Nos.60/680,491, 60/707,675 and 60/680,491.

Other barrier-forming corrosion inhibitors include dithiothiadiazole,alkyl dithiothiadiazoles and alkylthiols, alkyl being a saturated C₆-C₂₄straight hydrocarbon chain. This type of corrosion inhibitor canself-assemble on a metal surface to form a monolayer, thereby protectingthe metal surface from corroding.

According to another mechanism, a corrosion inhibitor binds more readilywith a corrosive element (e.g., H₂S) than with the metal nanowires.These corrosion inhibitors are known as “scavengers” or “getters”, whichcompete with the metal and sequester the corrosive elements. Examples ofH₂S scavengers include, but are not limited to: acrolein, glyoxal,triazine, and n-chlorosuccinimide. (See, e.g., Published U.S.Application No. 2006/0006120.) In certain embodiments, the corrosioninhibitor (e.g., H₂S scavengers) can be dispersed in the matrix providedits presence does not adversely affect the optical or electricalproperties of the GMR nanowire layer.

In other embodiments, the GMR nanowires can be pretreated with acorrosion inhibitor before or after being deposited on the substrate.For example, the metal nanowires can be pre-coated with abarrier-forming corrosion inhibitor, e.g., BTA and dithiothiadiazole. Inaddition, the metal nanowires can also be treated with an anti-tarnishsolution. Metal anti-tarnish treatments are known in the art. Specifictreatments targeting H₂S corrosion are described in, e.g., U.S. Pat. No.4,083,945, and U.S. Published Application No. 2005/0148480.

In yet other embodiments, the metal nanowires can be alloyed or platedwith another metal less susceptible to corrosive substances found in thenanowire's manufacturing or end-use environment. For example, addingchromium to the alloy layer should help prevent oxidation of nanowires.Likewise, using a copper gold alloy for the conductive layer shouldlessen the susceptibility to oxidation.

In still further embodiments, oxidation of the GMR nanowires introducedduring fabrication may be reduced or removed once the carrier fluid isapplied to and dried upon the substrate (depending on the nature of thematrix forming the GMR nanowire layer). Oxidation can be removed withmost acids including, HCL, acetic acid, or nitric acid. Alternatively,oxidation may be removed by placing the nanowires in a reducingatmosphere, e.g., hydrogen or nitrogen.

In certain embodiments, it will be desirable for the GMR nanowire sensorlayer to be substantially transparent. It will be understood that thetransparency of the nanowire layer will be affected by, among otherfactors, the loading of nanowires therein. Likewise, higher aspect rationanowires allow for the formation of a sensor with a less dense networkof nanowires leading to a lesser impact on optical properties. Exampleloading levels include about 0.05 μg/cm² to about 10 μg/cm², morepreferably from about 0.1 μg/cm² to about 5 μg/cm² and more preferablyfrom about 0.8 μg/cm² to about 3 μg/cm². These values depend strongly onthe dimensions and spatial dispersion of the nanowires.

Typically, the optical transparence or clarity of the GMR nanowiresensor layer can be quantitatively defined by parameters including lighttransmission and haze. “Light transmission” refers to the percentage ofan incident light transmitted through a medium. In various embodiments,the light transmission of the nanowire layer is between 50% and 98% (orany subrange there between). For a transparent sensor in which thenanowire layer is deposited or laminated on a substrate, the lighttransmission of the overall structure may be slightly diminished.Performance-enhancing layers, such as an adhesive layer, anti-reflectivelayer, anti-glare layer, may further contribute to reducing the overalllight transmission of the transparent nanowire layer. In variousembodiments, the light transmission of the transparent sensors can be atleast 50%, at least 60%, at least 70%, or at least 80% and may be ashigh as at least 91% to 92%.

In certain embodiments, adhesion agents may be added to enhance thestability and/or promote the adhesion of the matrix and the GMRnanowires. For example, an adhesion promoter (e.g., silanes) thatpromotes the coupling between organic matter and inorganic matter can beused. Examples of the silane-type adhesion promoters include GE SilquestA174, GE Silquest A1100 and the like. Antioxidants such as Ciba Irgonox1010ff, Ciba Irgonox 245, Irgonox 1035 can be used.

The GMR nanowire sensor layer may be formed by application of thecarrier solution to the substrate by any number of convention or futuredeveloped application techniques. Printing technologies are sometimesdivided between “sheet-based” and “roll-to-roll-based” approaches.Sheet-based techniques, such as inkjet and screen printing are best forlow-volume, high-precision work. Gravure, offset and flexographicprinting are more common for high-volume production, Thus, applicationtechniques may include sheet coating, high throughput web coating,laminating, spray coating, ink jet printer application, ink penapplication, metering rod application, micro-contact printing, vacuumfiltration, roll-to-roll application, line patterning or an airbrushtechnique. In particular, the carrier solution may be applied by anyprinted electronics technique.

Printed electronics techniques are a set of printing methods used tocreate electrical devices on various substrates. Printing typically usescommon printing equipment or other low-cost equipment suitable fordefining patterns on material, such as screen printing, flexography,gravure, offset lithography and inkjet. Electrically functionalelectronic or optical inks are deposited on the substrate, creatingactive or passive devices, such as thin film transistors or resistors.

Typically, the fabrication processes can be carried out usingconventional solution-processing equipment. Moreover, the fabricationprocesses are generally compatible with directly patterning the GMRnanowire layer. In the typical embodiment, the GMR nanowires beingapplied to a substrate in solution form results in the nanowires beingpositioned in a substantially random orientation when adhered to thesubstrate material. In other words, the nanowires will have a randomorientation in the x-y plane parallel to the substrate. The SEM image ofFIG. 15 showing a nanowire GMR thin film on PET substrate illustratesthis random orientation. FIG. 15 also suggests how in many embodiments,the GMR nanowires will lie substantially flat on the substrate,generally parallel to the x-y plane of the substrate (e.g., orientedperpendicular to the z-axis).

As an example of sheetcoating, GMR nanowires dispersion may be initiallydeposited onto the substrate. A roller is rolled across a top surface ofthe substrate, leaving a nanowires dispersion layer on the substrate topsurface. It is understood that a brush, a stamp, a spray applicator, aslot-die applicator or any other suitable applicator can be used in theplace of the roller.

Web-coating has been employed in the textile and paper industries forhigh-speed (high-throughput) coating applications. It is compatible withthe deposition (coating) processes for nanowire sensor fabrication.Advantageously, web-coating uses conventional equipment and can be fullyautomated, which dramatically reduces the cost of fabricating nanowiresensors. In particular, web-coating produces uniform and reproduciblenanowire layers on flexible substrates. Naturally, any of the processsteps described herein can be run on a fully integrated line or seriallyas separate operations.

In certain situations, the GMR nanowires may have the tendency toaggregate together into clumps and after time sedimentation of thenanowires occurs. To overcome this problem, ultrasonication of the GMRink (carrier fluid) may be used to help disperse the GMR nanowires andother additives more uniformly. Example ultrasonication proceduresinclude using an ultrasonication bath or an ultrasonication probe.During the ultrasonication process, the GMR nanowires may becomefragmented (break apart) or bent (curved) depending on the amount ofenergy and time used during the ultrasonication process.

In certain embodiments, an annealing step may be applied to GMR nanowirenetworks in order to promote fusion of the nanowires leading to betterelectrical conductivity. The nanowires may be placed in a furnacebetween 50° C. and 250° C. for approximately 10-180 minutes in either anair or reducing atmosphere environment. The temperature and time of theannealing process depends on the diameter, length, and chemical makeupof the nanowires. In addition, the density of nanowires in the network,additives, and substrate will affect the annealing process. Care shouldbe taken not to “melt” the multilayers of the GMR nanowires together,which may cause a significant decrease in the magnetoresistanceproperty. Naturally, the substrate should be of a material capable ofwithstanding the annealing temperatures.

Returning to FIG. 1, the position sensor 1 is formed by the magneticfield sensor 2 adhered to substrate 10 with conductors 7 and 8ultimately connecting the magnetic field sensor 2 to a detection circuit(which is explained further below). It will be understood that as magnet5 moves across a magnetic field sensor 2, the resistance is loweredthrough the GMR nanowire layer 3 and such lowered resistance may beregistered with the detection circuit. Obviously, the field of sensors 2may be arranged in any particular configuration. For example, FIG. 4illustrates how the position sensor may have a circular configurationwith conductor 7 being a formed in two semi-circular segments. Theembodiment of FIG. 4 suggests how the magnetic field sensor 2 can takeshapes other than serpentine lines 3. In FIG. 4, magnetic field sensor 2is a merely a circular or elliptically shaped layer of dried GMRnanowires which is in electrical contact with the conductors 7 and 8.

In certain embodiments, the sensor has a resistance of at least about0.5Ω/□ to 50 k Ω/□, where the “□” symbol represents unit area. In otherembodiments, the sensor may be characterized as having a resistancechange of at least about 0.01% per mT to 5% per mT (i.e., milli-Teslas).Typically, higher sensor sensitivity is achieved using some type of fluxconcentrator as described in more detail below. Certain sensorembodiments may have a magnetic saturation point of around 600 mT. Thesesensors have a 1.6% hysteresis error for a unipolar hysteresis test anda 9.1% hysteresis error for a bipolar hysteresis test.

A further example of a two-dimensional position sensor is seen in FIG.10. In this embodiment, a coversheet 15 with a series of indicia (imagesof different fruits in this illustration) is formed of some suitablethin nonconductive, nonmagnetic material. The coversheet 15 is thenposition on/over substrate 10 which has a series of magnetic fieldsensors 2 formed thereon in a two dimensional array. Although notexplicitly show in FIG. 10, it will be understood that conductors willbe attached to each of the sensors 2 and such conductors will beattached to a detection circuit. It can be seen how one or more of themagnetic field sensors 2 will be positioned under each indicia 16. Thus,when a magnet is placed over an indicia, at least one sensor 2 willexperience a change in resistance. In the particular embodiment of FIG.10, a ferrous material sheet 17 is positioned behind substrate 10 andwill provide a means for allowing a magnet to magnetically adhere to thesurface of the position sensor. It will be readily understood that auser (e.g., a shopper in a grocery store) wishing to indicate aparticular fruit (e.g., at an automated price checker) will simply movea magnet over the particular fruit indicia desired. Although FIG. 10suggests a flat two dimensional array, it may be envisioned that aflexible planar substrate could be applied to a curved surface, whichshould still be considered a two dimensional array for purposes of thisdisclosure. Nevertheless, those skilled in the art will recognizetechniques for employing the concepts described herein in threedimensional configurations.

As referenced above, the illustrated position sensor embodiments willtypically employ a detection circuit to identify when a magnetic fieldsensor 2 has encountered a change in magnetic field strength. Oneembodiment of such a detection circuit is seen in FIG. 12. Thisdetection circuit 24 illustrates a substrate 10 with an array 35 ofmagnetic sensors 2 (in this example, serpentine line magnetic fieldsensors 3) positioned on the substrate 10. An output voltage is formedfrom a voltage divider created by a magnetic field sensor 30 and amagnetic sensor 2 located in the array 35. A resistance change from eachmagnetic sensor in array 35 can be identified by using this detectioncircuit. One of the nine sensors 2 in array 35 may be selected to form avoltage divider with sensor 30 using multiplexers 25. Multiplexer 25 aconnects the sensor rows in array 35 to analog-to-digital converter 27and sensor 30. Sensor 30 terminates in voltage source 26. Multiplexer 25b connects the column of sensors in array 35 to ground 29. An outputvoltage is created between sensor 30 and one of the nine sensors 2located in array 35. Conductors 7 from individual sensors 3 willterminate in a first multiplexer 25 a while conductors 8 will terminatein a second multiplexer 25 b. The microprocessor 28 connects the correctmultiplexers channel to the multiplexers' input. An analog-to-digitalconverter 27 is positioned within the circuit in order to convert thevoltage from the voltage divider into a digital value to be communicatedto the microprocessor. Naturally those skilled in the art will recognizethat circuit 24 is just one possible detection circuit and manyalternative conventional and future developed detection circuits couldbe employed.

FIG. 5 illustrates an alternative position sensor arrangement. In thisembodiment, “push button” magnets 50 are positioned over one or more ofthe magnetic field sensors 2. When the magnets are depressed, they movecloser to the magnetic field sensors and reduce the resistance acrossthe GMR nanowire serpentine line segments 3. This allows a detectioncircuit to distinguish which of a plurality of push button magnets 50has been activated by a user. FIGS. 6A and 6B illustrate one example ofa push button magnet structure 50. This structure generally includes thehousing 52, the button 51 capable of moving upward and downward inhousing 52, an upper magnet 55, a GMR nanowire sensor layer 3, and alower magnet 56. A biasing mechanism will typically bias the magnet 55away from GMR nanowire sensor layer 3. In FIGS. 6A and 6B, the biasingmechanism is formed by magnets 55 and 56 being positioned in opposingpolar orientation, thereby having tendency to repel one another. Whenbutton 51 is depressed, magnet 55 moves closer to GMR nanowire sensorlayer 3 and changes the resistance across the sensor layer 3. When thedownward force on button 51 is removed, the opposing magnetic forcesreturns button 51 to its raised position. It will be understood thatwhile magnet 56 affects the resistance of GMR nanowire sensor layer 3,nevertheless, the movement of magnet 55 closer to sensor layer 3 causesa further measurable change in resistance. FIGS. 7A and 7B illustrate asimilar push button apparatus, but with a slightly different biasingmechanism. In this embodiment, the spring 58 is used to bias uppermagnet 55 away from GMR nanowire sensor layer 3, thus eliminating thelower magnet 56 seen in FIGS. 6A and 6B.

FIGS. 8A and 8B illustrate a still further embodiment of a push buttonassembly. In FIG. 8A, a cylindrical housing 52 has a flexible diaphragm60 formed across its top, with the button 51 attached to diaphragm 60'stop side and magnet 55 attached to its bottom side. Housing 52 is shownpositioned over a GMR nanowire sensor layer 3. The diaphragm 60 may beformed of any suitable elastic material allowing depression of button 51to move magnet 55 downward as seen in FIG. 8B and return magnet 55 toits upward position when the depressing force is removed. As describedabove, the pressing of button 51 is detected when the resistance acrossGMR nanowire sensor layer 3 is changed by the movement of magnet 55closer to the sensor layer 3. As one example, this push button assemblycould form the buttons described above in reference to FIG. 5. It willbe understood that that additional layers of material could be added tomany of the above embodiments. For example, a non-magnetic materiallayer can be between the magnets and sensors for FIGS. 1, 4, 5 in orderto protect the sensors or for aesthetic purposes.

In another embodiment, a magnetic potentiometer could be fabricated by athree-terminal resistor structure with a sliding magnet which forms anadjustable voltage divider as in FIGS. 9A to 9C. First resister 86 andsecond resister 87 are created from serpentine lines of solidified GMRcarrier solution. Both first resister 86 and second resister 87 areconnected to output terminal 89 while first resister 86 is alsoconnected to voltage source 88. It can be understood that as magnet 90changes its position relative to first resister 86 and second resister87, the respective resistance of those resisters also changes. Thiscreates the voltage divider effect, causing the voltage at output 89 tochange with respect to the position of magnet 90.

FIG. 11 suggests a still further component used in different embodimentsof the present invention. In FIG. 11, a carrier material 75 (e.g., apaper roll) is shown with a series of removable magnetic field sensors 2positioned thereon. The magnetic field sensors 2 will comprise anadhesive substrate material 70 (e.g., a paper “sticker” segment with anadhesive material holding it to carrier material 75) where the GMRnanowire sensor serpentine layer 3 is formed on the adhesive substratematerial. It can be visualized how these magnetic field sensors 2 may beremoved from carrier material 75 and adhered to another base to form anytype of sensor, including an array of sensors such shown in previousfigures or otherwise. It will be understood that conductors 7 and 8 andother elements of a detection circuit seen in FIG. 12 will be connectedto the array of sensors to create the desired configuration of positionsensor.

One modification to the above sensors could be the use of Wheatstonebridges. GMR materials themselves are very temperature dependent;however, simple arrangements, such as Wheatstone bridge circuits, can beapplied to eliminate the temperature dependence. Many commercial GMRsensors are configured in the Wheatstone bridge configuration todecrease the temperature dependence, reduce noise, and as well create alinear output for the sensor. A Wheatstone bridge configuration with twoactive GMR resistors and two shielded GMR resistors could be used toeliminate temperature dependence and create a linear output. Theresisters in the Wheatstone bridge circuit may be formed from serpentinelines of solidified GMR carrier fluid similar to the resister structuresseen in FIG. 9A. Of course, in none of the disclosed embodiments do theresisters need to be formed of serpentine lines, rather the resisterscould be formed by virtually any layer (regardless of shape) ofsolidified GMR carrier fluid.

While the above embodiments have been described in terms of positionsensors, many other types of devices could employ a similar type of GMRnanowire sensor layer. For example, magnetometers used to measure thestrength or direction of a magnetic field and output a signalaccordingly. A gradiometer may be used to detect magnetic fieldgradients found between GMR resistors in a Wheatstone bridgeconfiguration. Unlike the magnetometer, gradiometer devices areunshielded allowing all four resistors in the Wheatstone bridge to beactive. The gradiometer output can be bipolar versus unipolar and can beshaped by the use of a biasing magnet or flux guides.

The maximum current density carried by the GMR nano wire solidifiedlayer typically depends on the loading density of the nanowires in thelayer. Typically, the current through the nanowire layer will be lessthan 500 mA, and more preferably less than 100 mA, less than 50 mA, lessthan 10 mA, less than 1 mA, less than 0.5 mA, or less than 0.1 mA.

FIG. 13 illustrates another embodiment where the magnetic sensors may beused for ferromagnetic (metal) object detection. Normally when noferromagnetic object 65 is present close to the sensor 2, the magneticfield surrounding the sensor comes from a biasing magnetic field. Thebiasing magnetic field can come from any source 66 that produces amagnetic field, including permanent magnets, electromagnets, and theearth magnetic field. When a ferromagnetic object 65 comes into vicinityof the magnetic sensor 2, the magnetic field around the sensor isdistorted and the change in the magnetic field can be detected by thesensor.

In order to enhance the sensitivity of magnetic sensors, fluxconcentrators, can be used to concentrate the magnetic field in thepreferred direction across the magnetic sensors to achieve the desiredamount of sensitivity. FIG. 14 suggests one embodiment of a fluxconcentrator structure 80. Elongated pieces of soft magnetic material 81gather external magnetic flux and expose the sensor 2 to a magneticfield that is larger than the external magnetic field. Preferably, twopieces of soft magnetic material 81 of the same size are employed. Theconcentration factor is approximately the ratio of one fluxconcentrator's length to the gap between the two flux concentrators. Thelong dimension of the flux concentrators should butt up against thesensor structure. One example of a suitable soft magnetic material foruse in the flux concentrators is a permalloy. By employing fluxconcentrators, the sensitivity of magnetic sensors can be increased by afactor of 2 to 100.

Although the present invention has been described in terms of specificembodiments, those skilled in the art will recognize many variations andmodifications of those embodiments. For example, while most the abovedescribed embodiments deal with GMR nanowires, other nanowire types,e.g., TMR nanowires, CMR nanowires AMR nanowires, or OMR nanowires couldpotentially be used in alternative embodiments. All such variations andmodifications are intended to come within the scope of the followingclaims.

The invention claimed is:
 1. A magnetic position sensor comprising: a.at least one magnetic field sensor comprising a solidified layer of GMRnanowire carrier fluid formed on a substrate material, wherein thesolidified layer of carrier fluid comprises: i. discrete GMR nanowireseach having a diameter of less than about 0.5 μm and a length less thanabout 250 μm; and ii. a concentration of GMR nanowires in the solidifiedlayer being between about 0.001 and about 10 percent by weight of thecarrier fluid; b. a detection circuit capable of detecting a change inresistance of the magnetic field sensor; and c. a magnet allowingrelative movement closer to or further away from, the magnetic fieldsensor.
 2. The magnetic position sensor of claim 1, wherein the magneticfield sensor and the magnet are positioned in a housing and a biasingmechanism urges the magnet and magnetic field sensor apart.
 3. Themagnetic position sensor of claim 1, further comprising a plurality ofmagnetic field sensors positioned on the substrate in a two dimensionalconfiguration.
 4. The magnetic position sensor of claim 3, wherein aplurality of indicia are positioned over the substrate such that eachindicia corresponds to at least one magnetic field sensor, therebycausing a change in resistance in the at least one magnetic field sensoras the magnet is moved over the indicia.
 5. The magnetic position sensorof claim 1, wherein the solidified layer of carrier fluid has at leastabout 100 nanowires per mm².
 6. The magnetic position sensor of claim 1,wherein the GMR nanowires are positioned in a substantially randomorientation when adhered to the substrate material.
 7. The magneticposition sensor of claim 1, further comprising a corrosion inhibitorwhich is at least one of a barrier-forming corrosion inhibitor or ascavenger corrosion inhibitor.
 8. A GMR material carrying solutioncomprising: a. a carrier fluid; b. a concentration of discrete GMRnanowires contained in the carrier fluid, wherein: i. the discrete GMRnanowires each having a diameter of less than about 0.5 μm and a lengthless than about 200 μm; ii. the GMR nanowires have an aspect ratio of atleast five; iii. the concentration of GMR nanowires being between about0.005 and about 10 percent by weight of the carrier fluid.
 9. The GMRmaterial carrying solution according to claim 8, wherein the carrierfluid comprises 0.0001% to 0.5% of a surfactant; 0.01% to 15% of aviscosity modifier; and 80% to 99.0% of a solvent.
 10. The GMR materialcarrying solution according to claim 8, wherein the carrying solutioncomprises at least one solvent selected from group consisting of water,an alcohol, a ketone, an ether, a hydrocarbon or an aromatic solvent.11. The GMR material carrying solution according to claim 8, wherein thecarrying solution comprises at least one of a dispersion agent, aviscosity control agent, a corrosion inhibitor, or an adhesion agent.12. The GMR material carrying solution according to claim 8, wherein thediscrete nanowires each have a diameter of less than about 0.2 μm and alength less than about 100 μm.
 13. The GMR material carrying solutionaccording to claim 8, wherein GMR nanowires comprise alternatingferromagnetic and non-magnetic conductive layers.
 14. The GMR materialcarrying solution according to claim 8, wherein the carrier fluidincludes an ink, dye, or other pigment.
 15. The GMR material carryingsolution according to claim 8, wherein the carrier fluid issubstantially transparent.
 16. The GMR material carrying solutionaccording to claim 8, further comprising a flux concentrator agent. 17.The GMR material carrying solution according to claim 8, furthercomprising a corrosion inhibitor.
 18. A method of forming a magneticfield sensor comprising the step of adhering a solidified layer of GMRnanowire carrier fluid to a substrate material, wherein the GMR nanowirecarrier fluid comprises: i. discrete GMR nanowires in the carrier fluideach having a diameter of less than about 0.5 μm and a length less thanabout 200 μm; and ii. a concentration of GMR nanowires in the carrierfluid being between about 0.01 and about 10 percent by weight of thesolution; iii. wherein the GMR nanowires are positioned in a randomorientation when adhered to the substrate material.
 19. The method offorming a magnetic field sensor according to claim 18, wherein the GMRnanowire carrier fluid is adhered to the substrate by drying the carrierfluid on the substrate.
 20. The method of forming a magnetic fieldsensor according to claim 18, wherein the substrate is a flexible sheetof polymer material.
 21. The method of forming a magnetic field sensoraccording to claim 18, wherein the carrier fluid is applied to thesubstrate using at least one sheet based or roll-to-roll based printingtechniques.
 22. The method of forming a magnetic field sensor accordingto claim 18, wherein the sensor has a resistance change of about atleast 0.01% per mT.
 23. The method of forming a magnetic field sensoraccording to claim 18, wherein the sensor is formed in a predefinedshape on the substrate by applying the carrier fluid to a stamp havingthe predefined shape and then applying the stamp to the substrate inorder transfer the carrier fluid to the substrate.